Stringent emissions regulations such as those imposed by U.S. and European regulatory officials have progressively reduced the amount of diesel particulate matter (DPM) and other gaseous constituents allowed in the exhaust gases of diesel engines. The emissions levels proposed by the US07 and Euro 5 regulations are so low that they cannot be met without the use of exhaust aftertreatment devices. Diesel particulate filtration devices (DPF) and Diesel Oxidation Catalysts (DOC) are examples of devices which may be used to comply with particulate emissions levels.
DPFs filter the particulate matter from the exhaust gases to prevent them from exiting the tailpipe. After a period of operation, the collected particulates start to clog the filter. The filter either needs to be replaced or removed for cleaning, which is not practical, or it needs to be cleaned through a process known as regeneration. DPM is made up primarily of carbon, and is therefore combustible. Regeneration is a process where temperatures of the exhaust gases are made high enough to combust or oxidize the DPM within the filter. When engines are operated under higher loads the exhaust gas temperatures are generally high enough to cause at least some amount of regeneration without assistance. However, during light or short duration cyclic loads, or when ambient temperatures are low, the temperature of the exhaust gas is not high enough to produce regeneration. During these periods it is necessary to actively raise the exhaust gas temperature to facilitate regeneration or to increase exhaust gas temperatures to facilitate operation of other exhaust aftertreatment devices.
Methods for heating exhaust gas to a temperature sufficient for regenerating an aftertreatment device are known. For example, it is known to use a resistive electric heating element directly in the exhaust stream to increase exhaust gas temperature. It is also known to inject fuel into the exhaust and combust the fuel in a dedicated burner assembly to raise exhaust gas temperature. It is also known to inject a hydrocarbon into the exhaust gas and use a catalytic device that elevates exhaust gas temperature by catalytically oxidizing the injected hydrocarbon. An exhaust gas restriction device that applies an engine retarding load (braking load) to the engine can also be used to cause it to run at an elevated engine load condition, thus elevating the exhaust gas temperature. It is also known to elevate diesel particulate matter (DPM) temperatures by using microwaves.
The regeneration process can cause the temperature of the exhaust gas exiting the diesel particulate filter to be well in excess of 600° C. By comparison, normal operating exhaust temperature for a diesel engine depends on the load and can range from about 100° C. at idle to about 500° C. at high load.
At highway speeds, high exhaust temperatures do not usually pose problems because the relative high air speeds tend to dissipate the heat. Exhausting the higher temperature stream can create difficulties when the vehicle is stationary or moving at low speed and is near combustible materials. A truck typically has an exhaust stack pipe rising from the chassis adjacent to the truck cab. High temperature exhaust can produce a hot spot on the truck cab or trailer, or direct hot gases to a combustible substance, such as a building structure (for example, a loading dock or in a garage) or an overhanging tree.
It is desirable to provide an arrangement and a method for lowering the temperature of engine exhaust, particularly when the engine is operated at low loads to support regeneration of exhaust aftertreatment devices that does not involve complicated components or undue additional operating expense.
It is desirable at the same time to provide an arrangement that cools the exhaust gas exiting to the environment to avoid excess heat load on the surroundings.